What Are Superheat and Subcooling?
If you're an HVAC technician, these two numbers are the heartbeat of every refrigerant system you work on. They tell you whether the system is properly charged, whether the metering device is working, and whether the compressor is safe.
Superheat is the temperature of the refrigerant vapor above its saturation (boiling) point at a given pressure. It tells you how much the gas has been "superheated" past the point where it was fully evaporated.
Subcooling is the temperature of the liquid refrigerant below its saturation (condensing) point at a given pressure. It tells you how much the liquid has been "subcooled" below where it fully condensed.
Think of it this way — superheat protects the compressor (no liquid slugging), and subcooling indicates the charge level (is there enough refrigerant in the system).
Why Do They Matter?
| Measurement | Too Low | Normal Range | Too High |
|---|---|---|---|
| Superheat | Liquid flooding back to compressor — risk of slugging and compressor damage | 10–20°F (fixed metering) or 5–15°F (TXV) | Insufficient refrigerant reaching evaporator — low cooling, high discharge temps |
| Subcooling | System undercharged or restriction — flash gas before metering device | 8–14°F (TXV systems) | System overcharged — high pressures, wasted energy, potential liquid flooding |
The Formulas
Superheat Formula
Superheat = Actual Suction Line Temperature − Saturation Temperature at Suction Pressure
Superheat = T_suction_line − T_sat(P_suction)
Example (R-410A):
Suction pressure = 118 PSIG → Saturation temp = 40°F
Suction line temp = 52°F
Superheat = 52 − 40 = 12°F ✓ NormalSubcooling Formula
Subcooling = Saturation Temperature at Liquid Line Pressure − Actual Liquid Line Temperature
Subcooling = T_sat(P_liquid) − T_liquid_line
Example (R-410A):
Liquid pressure = 418 PSIG → Saturation temp = 115°F
Liquid line temp = 105°F
Subcooling = 115 − 105 = 10°F ✓ NormalInteractive Superheat & Subcooling Calculator
How to Use These Numbers for Diagnosis
Low Superheat + High Subcooling = Overcharged
The system has too much refrigerant. The evaporator is flooding and liquid may be reaching the compressor (slugging risk). The condenser is packed with liquid, driving subcooling up. Recovery is needed.
High Superheat + Low Subcooling = Undercharged
Not enough refrigerant. The evaporator is starving — the vapor gets excessively heated. The condenser doesn't have enough liquid to subcool. Check for leaks before adding charge.
High Superheat + Normal/High Subcooling = Restriction
Refrigerant is backed up before the metering device (subcooling looks fine) but can't flow through to the evaporator properly. Common causes: clogged filter drier, kinked liquid line, faulty TXV (stuck closed).
Never charge by superheat alone on a TXV system — the TXV controls superheat automatically. Use subcooling as your primary charging metric on TXV systems. For fixed metering (piston/cap tube), use the superheat method.
R-410A vs R-22 vs R-454B — Key Differences
| Refrigerant | Typical Suction PSI | Typical Liquid PSI | Status |
|---|---|---|---|
| R-22 | 65-80 PSIG | 210-260 PSIG | ⛔ Phased out (no new production) |
| R-410A | 110-130 PSIG | 350-450 PSIG | ⚠️ Being phased down (high GWP) |
| R-454B | 85-115 PSIG | 275-375 PSIG | ✅ Next-gen replacement (lower GWP) |
| R-32 | 100-130 PSIG | 325-425 PSIG | ✅ Used in mini-splits (mild flammability) |
Quick-Reference Target Ranges
| System Type | Target Superheat | Target Subcooling | Primary Charging Method |
|---|---|---|---|
| Fixed Metering (Piston/Cap Tube) | 10–20°F | N/A (varies) | Superheat method |
| TXV System | 8–15°F (auto-regulated) | 8–14°F | Subcooling method |
| Heat Pump (Heating) | 5–15°F | 5–10°F | Subcooling method |